Buried contacts are used in the fabrication of integrated circuits in order to establish current pathways through the underlying substrate rather than on the top surface of the circuit. Buried contacts ensure electrical isolation from other parts of the circuit, and leave the top surface free for use in establishing other connections and outside contacts giving access to the circuit.
For instance, in the fabrication of Static Random Access Memories (SRAMs), a buried contact is used to link the gate of one transistor to the drain of another in a paired transistor memory cell, as explained in U.S. Pat. No. 5,064,776 Roberts.
In the fabrication of Dynamic Random Access Memories (DRAMs), a buried contact is used between a storage capacitor and the source or drain of its controlling transistor.
Integrated circuit designers are forever trying to improve the conductive quality of buried contacts under the constraint of ever-increasing circuit complexity and demand for miniaturization.
A buried contact must retain a low resistive path, minimal current leakage to other parts of the circuit, and reduce the volume.
Buried contacts are typically created by diffusion or implantation of ions in the upper region of the circuit substrate through an opening in an insulating silicon oxide layer laid over the upper surface of the substrate.
The diffusion or implantation of the buried contact must extend beyond the periphery of the window open near or under a gate or storage capacitor structure in order to reach the source or drain region to which the gate or capacitor must be connected. Insufficient diffusion or implantation leaving too large a spacing between the edge of the buried contact and the outer edge of the polysilicon defining the source or drain may result in the creation of a parasitic MOS device having a relatively high threshold voltage (Vt) between the buried contact and the remote source or drain location. This parasitic MOS device may increase the buried contact resistance and degrade the circuit performance. The diffusion of the buried contact into the substrate is usually controlled by the size of the window, the type and duration of the diffusion or implantation process, and the judicious selection of doping elements.
Various techniques based on successive diffusion or punch-through implantations which have been used in the past require multiple masking steps which increase the fabrication complexity, processing time and cost.